SQL GetID

ALTER PROCEDURE [dbo].[Getid] @LastID NVARCHAR(6),
                              @NewID  NVARCHAR(6) output
AS
    DECLARE @Katalog NVARCHAR(40)
    DECLARE @i INT
    DECLARE @ID0 NVARCHAR(1)
    DECLARE @ID1 NVARCHAR(1)
    DECLARE @ID2 NVARCHAR(1)
    DECLARE @ID3 NVARCHAR(1)
    DECLARE @ID4 NVARCHAR(1)
    DECLARE @ID5 NVARCHAR(1)

    SET @Katalog = '0123456789abcdefghijklmnopqrstuvwxyz'

    IF @LastID = NULL
      BEGIN
          SET @NewID='000000'
      END
    ELSE
      BEGIN
          SET @ID0=SUBSTRING(@LastID, 1, 1)
          SET @ID1=SUBSTRING(@LastID, 2, 1)
          SET @ID2=SUBSTRING(@LastID, 3, 1)
          SET @ID3=SUBSTRING(@LastID, 4, 1)
          SET @ID4=SUBSTRING(@LastID, 5, 1)
          SET @ID5=SUBSTRING(@LastID, 6, 1)

          IF Charindex(@ID5, @katalog) = 36
            BEGIN
                SET @ID5='0'

                IF Charindex(@ID4, @katalog) = 36
                  BEGIN
                      SET @ID4='0'

                      IF Charindex(@ID3, @katalog) = 36
                        BEGIN
                            SET @ID3='0'

                            IF Charindex(@ID2, @katalog) = 36
                              BEGIN
                                  SET @ID2='0'

                                  IF Charindex(@ID1, @katalog) = 36
                                    BEGIN
                                        SET @ID1='0'

                                        IF Charindex(@ID0, @katalog) = 36
                                          BEGIN
                                              SET @ID1='0'
                                          END
                                        ELSE
                                          BEGIN
                                              SET @ID0=SUBSTRING(@Katalog, Charindex(@ID0, @Katalog, 1)+ 1, 1)
                                          END
                                    END
                                  ELSE
                                    BEGIN
                                        SET @ID1=SUBSTRING(@Katalog,Charindex(@ID1, @Katalog, 1 )+ 1, 1 )
                                    END
                              END
                            ELSE
                              BEGIN
                                  SET @ID2=SUBSTRING(@Katalog, Charindex(@ID2, @Katalog, 1 ) + 1,1)
                              END
                        END
                      ELSE
                        BEGIN
                            SET @ID3=SUBSTRING(@Katalog, Charindex(@ID3, @Katalog, 1 ) + 1, 1)
                        END
                  END
                ELSE
                  BEGIN
                      SET @ID4=SUBSTRING(@Katalog, Charindex(@ID4, @Katalog, 1) + 1,1)
                  END
            END
          ELSE
            BEGIN
                SET @ID5=SUBSTRING(@Katalog, Charindex(@ID5, @Katalog, 1) + 1, 1 )
            END

          SET @NewID=@ID0 + @ID1 + @ID2 + @ID3 + @ID4 + @ID5
      END